The narrator is happy to meet Fortunato because he has been planning to kill him for some time. The story begins with a statement for the narrator that "A Thousand injuries I have borne the best I could but when he ventured on insult, I vowed revenge." The narrator continues to say that he did not let on to Fortunato that he planned to kill him. Instead he continued to "smile" at his "friend" and never let Fortunato even know he was angry with him. However, when he comes across him at carnival time, Montresor has already sent out his servants and the trap is set. Thus when he meets Fortunato he says, "we are luckily met". The narrator is being sarcastic because what he is really saying is, "I'm so glad to see you because I've just set my trap for your death."
To answer this question, we must first consider the relationship between the narrator, Montresor, and Fortunato. Although Monstresor is careful to give Fortunato no "cause to doubt [his] good will," Montresor has a thirst for revenge due to the insults (again, unspecified) that Fortunato has allegedly hurled at him.
When Montresor runs into Fortunato at dusk one evening during the carnival season, he is thrilled to see him because he may now finally carry out the devious plan that he has been dreaming up: to lure Fortunato to his wine cellar under the precipice of tasting a fine Amontillado and to then murder him. Montresor puts on a good face so that Fortunato "did not perceive that to smile now was at the thought of his immolation."
This plan works because Montresor knows Fortunato's weakness for wine; he describes him as one of the few Italians with a "true virtuoso spirit." After luring Fortunato to his cellar and getting him drunk, Montresor paves a wall around Fortunato, effectively trapping the man so that he may suffocate to death in the damp belly of the cellar.